Atherosclerotic arterial disease is the leading cause of morbidity and mortality in the United States and most other developed countries. Atherosclerosis is a chronic disease process characterized by lipid deposits and fibrosis of the intima, irregularly distributed in large and medium sized arteries. The disease is progressive and most often becomes clinically manifested in the middle-aged and elderly. When severe, the atherosclerotic plaque causes a reduction of the cross-sectional area of the arterial lumen, with and without thrombosis. Resultant ischemic manifestations include angina pectoris, myocardial infarction, stroke, intermittent claudication, gangrene of the lower extremities and renovascular hypertension.
The current management of atherosclerotic disease includes preventative therapy aimed at minimizing known major risk factors such as hypertension, smoking, hypercholesterolemia and diabetes mellitus.
Coronary artery bypass grafting (CABG), carotid endarterectomy and bypass grafting (autogenous vein or synthetic graft) of the iliac, femoral and renal arteries are all well established surgical methods of palliative therapy. Although these procedures are often effective in relieving ischemia, each of these represents a major surgical operation with significant associated morbidity, mortality and expense. CABG, for example, requires the opening of the chest cavity (thoracotomy) and use of cardiopulmonary bypass, with not uncommon postoperative complications including postpericardotomy syndrome, Non-A Non-B hepatitis, stroke and a mortality of approximately one percent (1%)
Percutaneous transluminal angioplasty (PTA) by means of a balloon catheter is a relatively new "non-surgical" procedure with proven efficacy in relief of atherosclerotic obstruction of the coronary, renal and peripheral circulations. The technique involves the percutaneous passage (under local anesthesia) of a specialized balloon tipped catheter through the site of arterial narrowing, and inflation of the balloon to reduce obstruction. This is always done in conjunction with angiographic visualization of the vessel being treated. When successful, this procedure results in a reduction of the arterial stenosis and a decrease in the transstenotic pressure gradient. The mechanism of action is felt to consist of some combination of plaque compression, intimal splitting and medial/adventitial stretching. Healing of the balloon-damaged plaque may involve fibrosis and retraction of the split intimal elements, with further luminal enlargement in the weeks to months following the procedure.
The safety and efficacy of PTA is a function of the vessel being treated, patient selection, and the expertise of the physician performing the procedure. Primary angiographic success, defined as a 20% or greater reduction of stenosis, is now achieved in approximately 80-90% of attempts in carefully selected patients at experienced centers. The obvious advantage of PTA, compared to surgical palliative therapy, is that it does not require major surgery or general anesthesia with the associated sequelae.
Despite its proven efficacy in the palliation of obstructive atherosclerotic disease, PTA, as it is currently performed, has several important technical limitations. These limitations are particularly true in the application of PTA to the coronary circulation.
Even in the most skilled hands, dilation of an arterial obstruction is currently not achievable in approximately 20% of attempts. The most common cause of failed PTA is the inability to pass either the guidewire or dilating catheter through the site of a tight or eccentric stenosis. This problem is even more common in attempts to dilate the difficult to access right and circumflex coronary arteries. Although technical advances, such as steerable catheters, have reduced the frequency of unsuccessful attempts, inability to cross tight, eccentric or fully closed stenosis remains a major limitation of PTA.
Attempts at balloon or guidewire passage in vessels which are tightly stenotic may lead to arterial dissection and/or acute occlusion necessitating emergency vascular surgery. This major complication occurs in 6-8% of attempts at coronary angioplasty.
Inability to dilate an obstruction, even after proper balloon positioning and inflation is a second common mode of PTA failure. This problem is most frequently encountered in older plaques which are densely fibrotic and/or calcified.
Restenosis of the obstructed arterial segment following successful PTA is major problem with the current technique. This problem is more common following PTA of a coronary obstruction (30-35% at one year) than in the peripheral circulation (10-15% at two years). Pharmacologic attempts to reduce the incidence of restenosis have been largely unsuccessful.
Distal embolization of atherosclerotic plaque following balloon PTA occurs in approximately 5% of patients undergoing PTA of lower extremity or renal arteries. Although emboli are usually clinically insignificant in these vascular territories, such embolization could be catastrophic in the cerebral circulation. For this reason, balloon PTA is considered to be contraindicated for the treatment of obstructive lesions in the arteries of the aortic arch, such as the carotid artery.
DESCRIPTION OF THE PRIOR ART
In U.S. Pat. No. 4,207,874 (dated June 17, 1980) D. S. J. Choy describes a means for using a laser beam to tunnel though an arterial occlusion by vaporization of the obstruction. The difficulty with Choy's technique is that there is insufficient means to prevent simultaneous destruction of the arterial wall. For example, the Choy invention shows an intense laser beam aimed in the forward direction without significant beam attenuation. If the artery were to curve and the arterial wall were to be exposed to the laser beam, the wall could also be vaporized which could be catastrophic for the patient. Although the Choy patent describes a means for direct visualization of the obstructed region, it does not describe a centering means or a guidewire following means in order to guarantee that the laser beam does not illuminate part of the arterial wall. Furthermore, the Choy device may completely block a partially obstructed artery thereby cutting off blood flow to distal tissues for a significant time period. The result is ischemia which could cause irreparable damage to heart or brain tissue. Furthermore, if laser oblation was used in the carotid arteries, resulting gas bubble formation could cause some cerebral ischemia and resulting permanent brain damage.
In U.S. Pat. No. 4,273,128 (dated June 16, 1981) B. G. Lary describes a coronary cutting and dilating instrument used for opening a coronary stenosis that is restricting blood flow. The device described by Lary could not be used in a completely or nearly completely occluded artery because of its "blunt ovoid tip" nor could it pass through a very narrow stenosis. Furthermore, the Lary concept does not have any means to prevent its cutting blade from cutting through the arterial wall. Furthermore, there is no means taught in the Lary patent for centering the cutting blade within the arterial walls. Thus, if the probe wire 13 (FIG. 10) of the Lary patent guides the knife through a highly eccentric lumen within the stenotic plaque, its knife blade could cut through the arterial wall resulting in serious adverse effects for the patient.
Similar to the Lary device (although actually in a different field of use, namely removing growths from the teats of cows) is the device disclosed in U.S. Pat. 2,730,101 (dated Jan. 10, 1956) by R. D. Hoffman and entitled "Teat Bistoury with Expandable Cutter Knives." FIGS. 1, 2 and 3 of the Hoffman patent show an expansible cutter knife which can be inserted closed, opened within the teat, rotated to allow cutting of teat obstructions and then closed to withdraw the device from the canal. The Hoffman device has no means for preventing the blades from cutting the vessel wall, and hence if used within a human artery, such a rotating or oscillating blade would cut through the arterial wall. Because the flow of milk is out of the cow's body, particulate matter released during cutting with the Hoffman device would not harm the animal; however, in the entirely different field of use in the artery of a human, the absence of a definitive plaque collection means in the Hoffman device would result in the release of particulate matter into the flowing blood. Such particulate matter could then flow distally causing ischemia, stroke or even death. Thus the Hoffman device is entirely unsuitable for use in human (or animal) arteries.
Further advances are described in prior patent application Ser. Nos. 874,140 filed on June 13, 1986, and 694,746 filed on Jan. 25, 1985, both by Robert E. and Tim A. Fischell entitled "A Guide Wire Following Tunneling Catheter System for Transluminal Arterial Angioplasty", and "A Tunneling Catheter System for Transluminal Arteral Angioplasty", respectively. The '140 application describes a deice for removing stenotic plaque by advancing a tunneling catheter over a guidewire and within a guiding catheter. In this prior invention, the cutting is done by advancing the cutting catheter in a forward (antegrade) direction. The '746 application which is incorporated herein by reference, describes the use of a centering catheter which has expandable spokes to engage the inner arterial wall for centering the catheter. Plaque is removed by advancing a similar tunnelling catheter described in the '140 application. A potential difficulty in such a procedure is the inability to exert enough forward force to cut through a hard calcified plaque. Furthermore, if the tunneling catheter is advanced too far in the forward direction, it could cut the arterial wall. Even with the use of cutting (as opposed to fracturing the plaque which occurs with balloon dilation) there would still be the possibility of some particulate matter flowing into the bloodstream which could result in some distal ischemia.
Another application, Ser. No. 885,139 filed on July 14, 1986 by Robert E. and Tim A. Fischell and entitled "A Pullback Atherectomy Catheter System," describes the concept of first penetrating the stenotic plaque in a forward direction with a hollow conically pointed metal tip and then pulling the tip back in a retrograde direction. The tip, which includes a cylindrical cutting edge, is designed to shave off a cylindrical layer of the plaque as it is pulled back in the retrograde direction. Thus, the force required to perform the cutting is exerted by pulling back on the catheter (a retrograde motion) as opposed to cutting with a forward (antegrade) motion. The PAC utilizes sequentially larger diameter tips which progressively enlarge the lumen of the stenotic plaque.
PAC devices would typically be guided to the stenosis by a guidewire that is first passed through the narrowed lumen. Each one of the sequentially larger PAC tips is first advanced within a guiding catheter and over a guidewire until the tip passes through the stenotic plaque. The PAC tip is then pulled back to shave off plaque; then the PAC is withdrawn from the body. Each tip includes a chamber designed to collect the shaved off plaque thus preventing plaque particles from entering the bloodstream. Although PAC offers considerable advantage over prior atherectomy systems, it has three distinct disadvantages. Specifically, (1) the plaque collection chamber in the tip is both rigid and reasonably long which makes it difficult to use in highly curved arteries, (2) the tip diameter is limited to that diameter (approximately 3 mm) which could be readily inserted through a percutaneous guiding catheter passing through the femoral artery at the location of the patient's groin, and (3) to the extent that the plaque is not elastic, the cylindrical hole made in the plaque by the PAC tip moving in a forward (antegrade) direction precludes the removal of plaque when the tip is moved in the retrograde direction because the tip keeps the same diameter when moving in each of these two directions.